Raster Output Scanners (ROS) conventionally have a reflective, multifaceted polygon which is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction while the recording medium is being advanced in an orthogonal, or "process", direction. The beam or beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a binary sample stream representing the image to be printed, whereby the recording medium is exposed to the image represented by the samples as it is being scanned.
In a four layer printing process, typical for full color applications, the image to be printed will be divided into four color layers. Each layer is representative of one of four system colors, and is exposed on a different layer (in single station applications), or on a different photoreceptor (in multi-station applications).
For multiple beam printing, each beam may be intensity modulated with raster information which corresponds to one scan line of an interlaced representation of the image layer to be printed. Image layers processed for multiple-beam ROSs can be subdivided into a number of sub-images, depending upon the number of beams used in the ROS.
The sub-images, stored in channels, are further subdivided into scan lines. Each scan line represents one raster of image data. The number of scan lines per image layer is dependent upon factors such as resolution, spot size and image size. The number of scan lines reflected per facet of the polygon mirror equals the number of light sources (usually laser diodes) in the ROS, and is referred to as the beam set. Each rotation of the polygon, therefore, will lay down a number of scan lines per facet (corresponding to the number of laser diodes in the beam set) separated by a multiple of the scan spacing.
Interlacing is the introduction of additional scan lines between scan lines already laid down by a previous facet of the polygon. The number of scan lines interspersed between any two scan lines reflected by a single facet of the rotating polygon mirror is the interlace factor and has units of scans per beam. It is because of interlacing that each channel of the image data must be selectively delayed so that it is laid down sequentially on the photoreceptor even though the beam set does not lay down adjacent scans in the photoreceptor during a single facet of the polygon.
As the photoreceptor moves and the polygon rotates through a single facet, the beam set will lay down a number of scans separated by a multiple of the scan spacing. Successive facets will position successive scan lines between scan lines already laid down.
If there are q equally spaced beams in the beam set, then each laser beam could be modulated with information from every qth raster of an image, and in addition, since, in any rasterized image, there are q mutually exclusive sets of such rasters, each laser beam could be associated with its own set of rasters in each channel. This ensures that the superposition of the sets of rasters associated with each laser beam will define the entire image.
Typically, a start-of-scan (SOS) detector is utilized to synchronize the beams to the data rasters in the q channels, and q rasters are output for each scan. Since each scan of the polygon will produce q rasters, the recording medium will be advanced a distance corresponding to q times the reciprocal of the raster density, d, between each start of scan signal.
In order to synchronize the recording medium position with the beginning of the image rasters, a page-sync signal may be produced by a suitable means to signify a particular position of the recording medium (usually the top of page). After that time, but synchronized with SOS, raster information can begin to flow. Since there is generally an asynchronous relationship between page-sync and SOS, the initial image rasters will have a positional error in the process direction corresponding to the distance between q rasters on the photosensitive recording medium. For single layer printing, such as black and white, this may not be a problem, because an image registration error in the process direction of only q rasters may be unnoticeable. However, for multiple layer printing such as color printing, where layer registration is important, a positional error of q/d between layers is unacceptable.
In multiple layer printing applications, the top edge (i.e. top of page or page-sync) signal can occur when a laser diode is coincident with a scan placed by another laser diode in the previous layer. The need to divert data from the same memory channel that drove the laser on the previous layer into this new laser diode requires the channel-to-laser assignments to be switched, and the appropriate delays be generated. The situation is further complicated by the fact that the information, defining which laser diode is coincident with which channel is not available until the top edge synchronization (page sync) signal occurs, so there is little time to effect the change. Suitable means must be provided to measure the laser to channel error, redefine the laser to channel assignment and generate appropriate delays for each layer to be printed.
Therefore, it is understood that the raster positioning precision that is subject to a positioning error of q/d rasters for each layer scanned is unacceptable in multi-layer multibeam ROS printing. This error can be reduced to one raster by providing a system that allows for corrective mapping of memory locations and channel assignments of image data, with respect to the delay between the SOS and the page-sync for each layer to be printed.